1. Field of Invention
This invention relates to potentiometric measurements.
2. Prior Art Statement
The potentiometric determination of low levels of ion activity in a sample stream is limited by interference with electrode response caused by H.sup.+ and OH.sup.- ions. If high activities of ions are being measured, the interference of H.sup.+ and OH.sup.- ions is masked. At low activities, interference introduces substantial errors in measurement. Since H.sup.+ and OH.sup.- are universally present in aqueous sample streams, pH adjustment prior to electrochemical testing of low level cation and anion activities is essential to accurate determinations. More specifically, addition of acid lowers pH and removes OH.sup.- from the sample stream. Addition of base raises pH and removes H.sup.+ from the sample stream.
The prior art teaches that interference can be reduced by adjustment of pH. Changes in pH were made by the addition of acids or bases to the sample stream prior to electrochemical testing. The significant limitations of this method are related to the resulting dilution of the sample stream. For example, volumes added to the sample stream must be precisely metered in order to calculate original concentrations. Mixing of the sample stream and reagent must be complete, making mixing apparatus a must and creating time delays in potentiometric measurements. Real time monitoring of ionic concentrations is unattainable. For low level measurements of ion activities, dilution with a reagent may introduce an unknown amount of interfering, reactive, or even test ions in concentrations comparable to those in the sample stream.
An improved technique of pH adjustment involves the bubbling of an alkaline gas into the sample stream, thereby raising the pH. An instrument incorporating this technique is commercially available from Leeds and Northrup Company. Dimethylamine gas is bubbled at a constant rate into the sample stream in an absorber column, raising pH above 11 prior to an electrochemical determination of sodium activity. Notwithstanding that pumping is still necessary to ensure that the desired volumes of gas and sample stream are contacted, the Leeds and Northrup apparatus is an advance over the solution mixing method as it dispenses with the attendant problems of sample stream dilution.
The chief disadvantage of the apparatus is that contaminants are bubbled into the stream. Additionally, the absorber column is bulky and creates a delay in measurement, reducing the system's ability to monitor in real time.